In recent years, with the widespread use of portable electronic devices, such as mobile phones and notebook computers, there is a high demand for the development of small and light nonaqueous electrolyte secondary batteries having high energy density. There is also a high demand for the development of high-output secondary batteries as batteries for electric vehicles such as hybrid electric vehicles. Lithium ion batteries are nonaqueous secondary batteries that can satisfy these demands. A lithium ion secondary battery includes a negative electrode, a positive electrode, an electrolyte solution, and the like. Materials capable of sustaining lithium insertion and deinsertion are used as a negative-electrode active material and a positive electrode active material.
Lithium composite oxides, particularly, lithium-cobalt composite oxides, which are relatively easy to synthesize, are promising materials for use as the positive electrode material because lithium ion secondary batteries that use lithium composite oxides as the positive electrode material can achieve a high voltage of around 4V. As such, practical applications of lithium ion secondary batteries using lithium composite oxides are being developed as batteries having high energy density. Note that numerous efforts have been made to develop lithium ion secondary batteries using lithium-cobalt composite oxides with improved initial capacity characteristics and cycle characteristics, and various positive outcomes have been obtained therefrom.
However, because an expensive cobalt compound is used as a raw material in a lithium-cobalt composite oxide, the cost per capacity of batteries using lithium-cobalt composite oxides is substantially higher than nickel-hydrogen batteries, and as such, their applications are substantially limited. Thus, cost reduction of the positive electrode active material is desired with respect to both small secondary batteries used in portable devices and large secondary batteries for electric power storage and electric vehicles, and the development of techniques for reducing the cost of the positive electrode active material to enable production of a more inexpensive lithium ion secondary battery will have great potential and industrial significance.
An example of a potential new material to be used as the active material of a lithium ion secondary battery includes a lithium-nickel composite oxide that uses nickel, which is a cheaper alternative to cobalt. The lithium-nickel composite oxide exhibits a lower electrochemical potential as compared with the lithium-cobalt composite oxide, and as such, the lithium-nickel composite oxide may be less prone to problems of decomposition due to oxidation of the electrolyte, achieve higher capacity, and exhibit a high battery voltage comparable to that of the cobalt-based lithium ion secondary battery. As such, active research and development efforts are being made with respect to the lithium-nickel composite oxide. However, when a purely nickel-based lithium composite oxide synthesized with only nickel is used as the positive electrode active material of a lithium ion secondary battery, cycle characteristics may be degraded as compared with cobalt-based lithium ion secondary batteries. Also, such a purely-nickel-based lithium ion secondary battery may be prone to battery performance degradation when stored or used in a high temperature environment. In this respect, lithium-nickel composite oxides obtained by substituting a part of nickel with cobalt or aluminum are generally known.
A general method for producing the positive electrode active material involves (1) first, preparing a nickel composite hydroxide as a precursor of the lithium-nickel composite oxide using the so-called neutralization crystallization method, and (2) mixing the precursor with a lithium compound and firing the mixture. Of the above process steps, a representative example of process step (1) for producing particles by the neutralization crystallization method includes a process using an agitation tank.
Patent Document 1 describes how the shear force generated by agitation affects the growth of nickel hydroxide particles and how the average particle diameter of the nickel hydroxide particles decreases as the shear force gets stronger such that an impeller for controlling the shear force is necessary.